1. Field of the Invention
The present invention relates to disposal of sludge and in particular relates to disposal of sludge in geologic formations.
2. Background Art
Each year almost 2 million dry tons of sewage sludge are created in the coastal and estuarine region of the United States. Almost three times this amount is created each year in the nation, and the amount is increasing. In addition to containing abundant nutrients, sewage sludge commonly also contains a considerable amount of various toxic components that are concentrated in this final residue of sewage treatment.
Presently, the main choices for disposal of the sewage sludge are by dumping either on land or at sea. Dumping on land runs the risk of allowing toxic components to find their way into fresh water resources. In near coastal areas, both toxic components and nutrients may leach and flow into and through the estuarine zone, contributing their deleterious effects in this area.
Dumping at sea runs a multiple risk: toxic components can be concentrated through the food chain, to affect the health of marine life and the quality and safety of seafood. Excess nutrients and oxidizable material can deplete the oxygen content of seawater where oceanographic conditions permit. Human disease pathogens, when present in the sludge, can directly contaminate seafood. Persistent excess nutrients may possibly contribute to blooms of toxin producing marine microorganisms, and these toxins will be concentrated in dangerous amounts in seafood such as shell fish.
There is nothing particularly evil about human excrement or sewage based solely thereupon. It is excellent and nutritious for the right organisms and makes fine fertilizer, provided human diseases are not recycled with it. The problem of disposal of materials found in sewage systems resides in the other materials that are introduced into sewage systems. For example, industrial waste and toxic chemicals often are disposed of in sewage systems. These materials create the intransigent problems associated with safe sewage disposal. Thus, it would be desirable to have a method of sewage disposal that provides substantial environmental isolation of the disposed of sewage.
Aside from direct dumping, incineration of sewage sludge has been contemplated. Incineration of sewage sludge, however, leaves a residue in which toxic materials, especially metals, are even further concentrated. Dumping of such residues is especially environmentally undesirable.
The patent literature discloses methods for disposing of sludge underground. More specifically, U.S. Pat. No. 3,335,798 of Querio et al discloses a process for disposal of biologically activated waste sludge in a well drilled in a land formation. The land formations are disclosed as being "porous and permeable and readily accept the waste solids-containing slurry". The sludge is in the form of an aqueous slurry of 1 to 5 percent undissolved sewage solids. The hole is at a depth of 3916 feet, and the bore of the well is 81/2 inches. In evaluating the Querio et al patent, one should consider the desirability of having waste materials substantially permeate a geologic formation in which they are deposited. One should also consider the availability of such holes and geological conditions in areas of high population and need. Such a practice might be highly questionable because of the risks of environmental contamination by unknown migration or unwanted exit. Rather, it would be desirable to provide a method for disposing of sludge in a low-permeability formation to provide environmental isolation and in a controlled manner.
Furthermore, the example of drilling an 81/2 inch hole to a depth of 3916 feet requires large amounts of time (perhaps days or weeks), energy, and expense and provides a disposal site for only a relatively small amount of sewage solids. For such a hole, assuming an approximately 200 feet sealing space on top of the disposed of waste, the remaining 3716 feet deep hole would be capable of holding only approximately 0.6 ton of sewage solids, counting only the volume of such a hole, itself and not the uncontrolled volume into which the slurry would be injected. It would be desirable, therefore, to provide a sewage sludge burial method that is capable of handling large amounts of sludge in a relatively short period of time, with relatively low energy expenditure, and without uncontrolled and unknown invasion and injection.
U.S. Pat. No. 3,724,542 of Hamilton discloses a method of disposal of waste activated sludge which involves using deep wells which penetrate hydrocarbon formations. The sludge is introduced into the formation through the well bore. Methane is produced by the organisms in the sludge as they interact with the hydrocarbons in the formation. It would be desirable, however, to dispose of sludge in a relatively shallow hole in a formation that does not contain significant hydrocarbons, and also by a method that does not require uncontrolled and relatively unknown invasion of the formation by injection and fracturing.
Besides sewage sludge, certain soils from former industrial or disposal sites may contain toxic components, the cleanup of which is made difficult without a safe method of disposal. Also, dredged material from some harbors, bays, lakes, estuaries, or channels may have sufficient contamination to make it environmentally unwise to simply shift the material to some other unprotected and poorly isolated location.